While extrusion and injection molding machines are used for resin molding, it is sometimes necessary to clean the interior of the machine. For example, after a molding machine is used to mold a certain material, the molding material is changed and molded in the same machine. In this case, some of the previous molding material is left in the molding machine. Thus, if the new material is molded in the molding machine as such, the residue is incorporated in the new material as impurity. Molded parts of the new material are deteriorated in quality or even become outside the quality standards. For this reason, the interior of the molding machine must be cleaned before the molding material is changed.
A problem arises even when a common molding material is continuously molded. If part of the material stagnates within the machine for a long time without being discharged out, that material can be carbonized by the thermal history during the time, indicating that carbide is left in the molding machine. If such carbide resides in the molding machine, problems arise as in the above case, for example, molded parts are deteriorated in quality. Therefore, even in continuous molding of a common material, the molding machine is desirably cleaned at regular intervals.
In the prior art, cleaning of the molding machine interior is implemented by disassembling the machine. Since the disassembly of the machine is time and labor consuming and causes a substantial reduction of productivity, it would be desirable to develop a purging agent with which the molding machine interior can be cleaned simply by introducing the agent into the machine and performing ordinary operation. For example, the following purging agents are known in the art.
Patent Document 1 discloses a molding machine purging composition comprising 70 to 95% by weight of a thermoplastic resin and 30 to 5% by weight of glass fibers. Patent Document 2 describes a molding machine purging agent comprising a thermoplastic resin and a fibrous filler having a length of at least 2 mm such as glass fibers and carbon fibers. Patent Document 3 describes that a thermoplastic resin composition comprising a thermoplastic resin, an organic filler of a specific size such as wood meal, and a lubricant is an effective purging agent for molding machines.
However, these purging agents are still unsatisfactory. While inorganic fillers such as glass fibers are used in Patent Document 1, they are known to function to abrade internal components of the molding machine. If the purging agent with a high loading of inorganic filler is used for a long period of time, screws, cylinders and other components can be abraded. The purging agent of Patent Document 2 also has a risk that screws, cylinders and other components can be abraded since glass fibers are used as in Patent Document 1. The purging agent of Patent Document 3 contains wood meal, which is not melted upon heating. At the end of purging operation, wood meal is left in the molding machine along with the thermoplastic resin, and removal of the wood meal and resin is quite difficult.